linux-kernel - Re: [PATCH v2] usb: typec: qcom-pmic-typec: split HPD bridge alloc and registration

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Date: Mon, 8 Apr 2024 14:48:44 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Johan Hovold <johan@...nel.org>
Cc: "Bryan O'Donoghue" <bryan.odonoghue@...aro.org>, 
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>, Bjorn Andersson <andersson@...nel.org>, 
	Konrad Dybcio <konrad.dybcio@...aro.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	linux-arm-msm@...r.kernel.org, linux-usb@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Caleb Connolly <caleb.connolly@...aro.org>
Subject: Re: [PATCH v2] usb: typec: qcom-pmic-typec: split HPD bridge alloc
 and registration

On Mon, 8 Apr 2024 at 14:44, Johan Hovold <johan@...nel.org> wrote:
>
> On Mon, Apr 08, 2024 at 01:49:48PM +0300, Dmitry Baryshkov wrote:
> > On Mon, Apr 08, 2024 at 09:11:32AM +0200, Johan Hovold wrote:
> > > On Mon, Apr 08, 2024 at 04:06:40AM +0300, Dmitry Baryshkov wrote:
> > > > If a probe function returns -EPROBE_DEFER after creating another device
> > > > there is a change of ending up in a probe deferral loop, (see commit
> > > > fbc35b45f9f6 ("Add documentation on meaning of -EPROBE_DEFER").
> > > >
> > > > In order to prevent such probe-defer loops caused by qcom-pmic-typec
> > > > driver, use the API added by Johan Hovold and move HPD bridge
> > > > registration to the end of the probe function.
> > >
> > > You should be more specific here: which function called after
> > > qcom_pmic_typec_probe() can trigger a probe deferral?
> > >
> > > I doubt that applies to tcpm->port_start() and tcpm->pdphy_start() in
> > > which case the bridge should be added before those calls unless there
> > > are other reasons for not doing so, which then also should be mentioned.
> > >
> > > I suspect the trouble is with tcpm_register_port(), but please spell
> > > that out and mention in which scenarios that function may return
> > > -EPROBE_DEFER.
> >
> > The probe loop comes from from tcpm_register_port(), you are right.
> > However then putting bridge registration before the _start() functions
> > is also incorrect as this will be prone to use-after-free errors that
> > you have fixed in pmic-glink.
>
> You obviously have to mention that in the commit message as that is a
> separate change and also one that looks broken as you're now registering
> resources after the device has gone "live".

No. I'm registering a child device rather than a resource.

> So you also need to explain why you think that is safe, if it should be
> done at all. You're essentially just papering over a DRM bug in the
> unlikely event that probe fails.

Unfortunately, as pointed out by Reported-by, Caleb has actually hit
the probe failure loop.

-- 
With best wishes
Dmitry